1. The Field of the Invention
The present invention relates generally to color shifting pigment compositions. More specifically, the present invention relates to optically variable pigment flakes, foils, and colorants which exhibit enhanced color shifting properties with changes in angle of incident light or with viewing angle.
2. The Prior State of the Art
Various optically variable pigments, also known as color shifting pigments, have been developed for a wide variety of applications. For example, color shifting pigments have been used in applications ranging from automobile paints, to anti-counterfeiting inks for security documents and monetary currency. Color shifting colorants are produced by dispersing small flakes, which are typically composed of multiple layers of thin films having particular optical characteristics, throughout a medium such as paint or ink that may then be subsequently applied to the surface of an object. Color shifting pigments and colorants exhibit the property of changing color upon variation of the angle of incident light, or as the viewing angle of the observer is shifted.
Conventional optically variable pigments are typically based on multilayer thin film interference structures. For example, U.S. Pat. No. 5,135,812 to Phillips et al. discloses optically variable thin film flakes having several different configurations of layers, such as transparent all-dielectric stacks, or transparent dielectric and semi-transparent metallic layered stacks. In U.S. Pat. No. 5,278,590 to Phillips et al., a symmetric three layer optical interference coating is disclosed which comprises first and second partially transmitting absorber layers which have essentially the same composition and thickness, and a dielectric spacer layer located between the first and second absorber layers.
Color shifting platelets for use in paints are disclosed in U.S. Pat. No. 5,571,624 to Phillips et al. These platelets are formed from a symmetrical multilayer thin film structure in which a first semi-opaque layer is formed on a substrate, with a first dielectric layer formed on the first semi-opaque layer. A reflecting layer is formed on the first dielectric layer, followed by a second dielectric layer formed on the reflecting layer, and a second semi-opaque layer formed on the second dielectric layer. The color shifting properties of a pigment can be controlled through proper design of the optical coatings used to form the flakes. Desired effects can be achieved through the variation of parameters such as the thickness of the layers forming the flakes and the index of refraction of each layer. The changes in perceived color which occur for different viewing angles or angles of incident light are a result of a combination of selective absorption of the materials forming the layers, and wavelength-dependent interference effects. The interference effects, which arise from the superposition of light waves that have undergone multiple reflections, are responsible for the shifts in color perceived with different angles. The reflection maxima changes in position and intensity, as the viewing angle changes, due to the absorption characteristics of a material which are selectively enhanced at particular wavelengths from the interference phenomena.
It is an object of the present invention to provide optically variable pigment flakes and foils which exhibit enhanced color shifting properties.
Another object of the invention is to provide color shifting pigment flakes and foils which have substantial shifts in chroma and hue with changes in angle of incident light or viewing angle.
It is yet another object of the invention to provide color shifting flakes which may be easily and economically utilized in colorants such as paints and inks for various applications.
To achieve the forgoing objects, and in accordance with the invention as embodied and broadly described herein, pigment flakes are provided which can be used to produce colorant compositions having color shifting properties. The pigment flakes can have a symmetrical coating structure on opposing sides of a reflector layer, can have an asymmetrical coating structure with all of the layers on one side of the reflector layer, or can be formed with one or more encapsulating coatings around a reflector layer. The coating structure includes a selective absorbing layer on one or more sides of the reflector layer, a dielectric layer on the selective absorbing layer, and an absorber layer on the dielectric layer. The pigment flakes exhibit a discrete color shift such that the pigment flakes have a first color at a first angle of incident light or viewing and a second color different from the first color at a second angle of incident light or viewing. The pigment flakes can be interspersed into liquid media such as paints or inks to form colorant compositions for subsequent application to objects or papers.
A nonsymmetrical foil is also provided which includes the coating structure of a reflector layer, a selective absorbing layer on the reflector layer, a dielectric layer on the selective absorbing layer, and an absorber layer on the dielectric layer.
These and other aspects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.